Mental illness refers to a behavioral pattern which causes difficulty or impairment in personal functioning, which can occur as single, remitting or persistent episodes. There are various types of mental illnesses, which include mood disorder, psychotic disorder, personality disorder, eating disorder, sleep disorder, substance use disorder, developmental disorder and anxiety disorder. Substance use disorder (SUD) is a type of mental illness, characterized by continued use of drugs and alcohol despite significant harm and negative effects. They are associated with a variety of behavioral, mental, emotional and physical problems, including, chronic guilt, physiological withdrawal symptoms and an inability to decrease or quit consuming the substances despite recurrent attempts. There are various categories of drugs with which a person can be involved, such as tobacco, opioids, phencyclidine, hypnotics, cannabis, inhalants, and stimulants. This book explores all the important aspects of mental illness and substance use disorders in the present day scenario. It presents researches and studies performed by experts across the globe. Those in search of information to further their knowledge will be greatly assisted by this book.